Luminescent or scintillating monocrystalline materials, such as YAG (yttrium aluminium garnet, Y3Al5O12), YAP (yttrium aluminium perovskite, YAlO3), YSO (yttrium orthosilicate, Y2SiO5), LYSO (lutetium yttrium orthosilicate, {Lu,Y}2SiO5), LPS (lutetium pyrosilicate, Lu2SiO7) and sapphire (Al2O3), doped with ions of rare earth elements (e.g. Ce, Pr) or certain transition metals (e.g. Cr, Zr, Ti), very often show high refractive indices, frequently exceeding the value of 1.8, and luminescence in the ultraviolet (UV), visible and near-infrared regions of the electromagnetic spectrum.
These monocrystalline materials can be used for instance as scintillators in the detection of ionising radiation such as gamma rays, X-rays, beta- or alpha-particles, or extreme ultraviolet (E-UV) or UV radiation. They can also be used in light conversion elements in optoelectronics, especially in the field of LED's, where they are known as “luminophores”.
Luminescence in certain materials, however, may occur also by other mechanisms apart from photoluminescence, such as electroluminescence, thermoluminescence, mechanoluminescence, and others.
Such monocrystalline materials may thus be generally termed “radiation converting materials” so as to encompass all such applications.
The converted radiation output from such monocrystalline materials is limited by the difference between the refractive indices of a low-index ambient environment, e.g. typically air or a vacuum, and the high-index monocrystalline material itself. The resulting maximum angle at which the emitted or converted light can still leave the monocrystal is subject to Snell's law. For materials with a refractive index of 1.8, the maximum output angle of the ray of light incident on the exit surface of a plate of the material which can still exit the plate is 33°45′ (measured from the surface normal). This means that only a minor proportion of the radiation impacting on the plate exit surface can be extracted from it into the ambient environment. Moreover, this value is further reduced if one takes into consideration the fact that the intensity and spatial distribution of outgoing radiation further depends on the application, the size of the body of monocrystalline material and the method of excitation of the radiation.
When such monocrystalline materials are exposed to exciting radiation (and/or subjected to other luminescence mechanisms), scintillation—i.e. emission from the luminescent centres within the structure—begins, and occurs uniformly in all directions. When required to detect high-energy radiation, such scintillators are frequently used in the form of thin monocrystalline slices polished in order to achieve the best possible spatial resolution. In light convertors, e.g. in LED applications, the luminophore is typically also used in the form of a thin plate or platelet.
As a consequence of their high refractive index, the thin slices of the monocrystalline material show a significant light-guiding effect being the phenomenon of total internal reflection (TIR), in which a large proportion of the radiation is disseminated by multiple reflections at the material-environment interface into the body or side regions of the monocrystalline material, from which it subsequently exits to the environment in one or more directions which are undesirable. Emission of the radiation from such regions or in such directions is undesirable because it can be considered as a loss for practical application purposes. Moreover, with increasing distance that the converted radiation has to travel inside the monocrystalline material, the probability of self-absorption occurring in the heterogeneities and impurities contained therein increases.
The currently widely used monocrystalline materials of a doped silicate type, e.g. Y2SiO5:Ce or materials based on yttrium aluminium garnet (Lu3Al5O12:Pr) or perovskite materials (YAlO3:Ce), generally show high refractive indices (namely 1.78, 1.84 and 1.95 respectively). As mentioned above, this results in extraction of light from the material-air interface with only a low light output, and a large proportion of the light is reflected from the interface back into the material by TIR. Moreover, such limitations on the light output can cause a low output even in newly developed materials with excellent luminous yields exceeding those of materials currently in commercial use.
Furthermore, radiation which is converted by a luminophore generally is omnidirectional in comparison with the light generated by an LED device. This effect can cause undesirable in homogeneous distributions of radiated light and can also cause problems in viewing angle and problems with light collimation.
In the current state of the technology, all the types of luminescors or scintillators described above have a disadvantage in that the high refractive index of the material and their shapes or layout geometries are such that most of the energy of the light emitted by them is lost through internal reflections, i.e. it is not propagated in a given desired direction, and it may even be eventually radiated out of the luminescor/scintillator body in unwanted or undesirable directions. Therefore, such light sources are very often ineffective; in some cases the typical value of useful radiation emitted from the luminophore can be as little as below 10% of the input radiation energy.
In some known examples of LED technology in this field, such as disclosed in WO 2009/126272A1, WO 2014/173376A1, US 2014/0106488A1 and US 2008/0128730A1, the excitation surface of the luminophore may be polished, textured or treated chemically in order to increase the output level of the emitted radiation. However, this solution at best only partly addresses the problems discussed above, since it only deals with the issue of increasing the amount of radiation emitted; any directing of the output radiation in particular ways is not possible using these known configurations.
In other known art, radiation from an LED can also be extracted using photonic crystals (e.g. as disclosed in WO 2012/108627A2 and WO 2008/060594A2) or thin anti-reflective layers (e.g. as disclosed in US 2012/0313120A1 and US 2004/0188696A1), or by shaping of the luminophore itself in a similar manner to production of hemispheres or spherical caps from a silicone gel over the luminophore in LED technology (e.g. as disclosed in EP 2650934A1), or from silicone gel with dispersed luminophore (e.g. as disclosed in EP 2202444A1). The luminophore can also be worked into other rotationally symmetrical shapes, such as a truncated cone, spherical cap, etc. This may ameliorate the total internal reflection effect to some degree, but production of shapes other than planar slices in a large-batch production context is difficult and costly, particularly when considering the high hardness of these materials (˜8 Moh).